Process for desulphurizing mercaptan-bearing oil



Patented Apr. 23, 1935 UNITED STATES PROCESS FOR DESULPHURIZING MER- CAPTAN-BEARING OIL Lovell V. Chaney and Walter A. Schulze, Battlesville, 0kla., assignors to .Phillips Petroleum Company, Bartlesville, 0kla., a corporation of Delaware No Drawing. Application May 10, 1932, Serial No. 610,481

3 Claims.

The present invention relates to a novel and improved method of desulphurizing hydrocarbon oils and the like. More specifically, the invention has especial relation to the elimination of mercaptans from petroleum oils, liquefied petroleum gas, or the like.

A primary object of the invention is the removal of mercaptans from mercaptan-bearing material by means of a solvent therefor, followed by regeneration and recovery of the solvent from the resultant mercaptide-saturated solution, whereby repeated use may be made of the same solvent which is thus salvaged instead of being discarded.

A further object of this invention is the provision of a continuous process for the removal of mercaptans from petroleum oils or the like by extraction thereof from the oil or gas by means of an alkaline solution, which process shall include the step of oxidizing the mercaptides formed in the extracting solution, whereby the latter may be freed therefrom and is adapted for reuse in the extraction of additional quantities of mercaptans.

Another object is to furnish a process for causing higher mercaptides to be removed by hydrolysis and volatilization. In other words, the lower molecular weight mercaptides in the extracting solution are oxidized to disulphides by molecular, or free, oxygen and may be removed by dissolving them in a mercaptan-free gas oil or other petroleum fraction by bringing the two into intimate contact. The higher molecular weight mercaptides hydrolyze and during the aeration or contact with free oxygen are volatilized and thereby removed from the caustic solution.

Other objects of the invention will become apparent from the detailed disclosure of the invention hereinafter set forth.

It is well known that sulphur compounds are present in practically all unrefined petroleum distillates and, also, that in many cases, the sulphur content is so high that the petroleum product must be subjected to severe refining methods in order to reduce the sulphur content to specifications.

Strong sulphuric acid is generally used for such refining operations, but there are certain inherent objections to this method, particularly in the treatment of cracked distillates, which entails enormous losses of unsaturated hydrocarbons due to polymerization, and the consequent increase of the knocking tendency of the petroleum oil. Other methods have been proposed, but none is in general use at the present time.

In contradistinction to known methods, the present invention provides a simple and efficient process for bodily extracting the mercaptans from petroleum oil, gas, or liquefied petroleum gas, whereby the total sulphur content of the petroleum product is lowered by an equivalent amount. The mercaptans in the distillates from certain crudes constitute a relatively large fraction of the total sulphur content of the distillate, and in these cases the reduction in the sulphur content by treatment according to this invention is considerable. The present process is also applicable in cases where it is desired to lower the sulphur content only 0.01 or 0.02%.

According to the present invention, mercaptans are extracted from petroleum products, petroleum naphtha for example, by treatment with an aqueous alkaline solution such as NaOH solution. It is found, however,- that the solution soon reaches a more or less steady state of saturation and fails to remove additional quantities of mercaptans. This equilibrium, of course, depends upon the distribution coeflicients of the mercaptans between theNaOH and the naphtha.

The term saturated is used to imply a state of ineffectiveness, i. e., the alkaline wash solution is said to be saturated" when it will no longerremove a considerable part of the mercaptans present. Therefore, the saturation point in any given case will depend on (1) the particular mercaptan or mixture of mercaptans present, (2) the percentage of mercaptans present, and (3) the amount of sulphur reduction desired. The alkaline solution, however, cannot be discarded at this point, as the cost thereof is too great.

According to the present invention, a method is provided for regenerating the mercaptidesaturated alkaline solution, thus making it possible to use it over and over again. In practicing our process, the sodium hydroxide solution, or other suitable alkaline solution, is commingled with the oil or gas, preferably by passage in countar-current relation thereto. When the extracting solution becomes spent, it is removed to a separate tank for regenerative treatment.

We have discovered that the sodium hydroxide solution containing the mercaptides can be completely regenerated by subjecting the solution at elevated temperatures to a stream of air and/or other oxidizing gas. Two concurrent reactions are involved in the regenerating process, (1) oxidation of mercaptides to disulphides, and (2) removal'of free merceptans due to hydrolysis oi. mercaptides. These reactions are exemplified by the following equations:

.Our investigations reveal that sodium methyl mercaptide, for example, is rapidly oxidized by a stream of air and that sodium n-butyl mercaptide is much more slowly oxidized. On the other hand, sodium methyl mercaptide is only slightly hydrolyzed while sodium n-buty'l mercaptide is hydrolyzed to a considerable degree. In other words, the rate of oxidation varies inversely with the molecular weight of the mercaptans, while the degree of hydrolysis varies directly with the molecular weight of the mercaptans.

We have taken advantage oi these properties of the mercaptides in our sodium hydroxide regeneration method. Petroleum oils or gases usually contain a mixture of mercaptans. These are extracted by the sodium hydroxide, as hereinbeiore described, and the sodium hydroxide is then regenerated by means of a stream of air and/or other oxidizing gas. The lower mercaptides are rapidly oxidized to disulphide form and the higher mercaptans are volatilized from the solution. In this manner, the caustic solution may be completely regenerated in about ten minutes at a temperature somewhat under the boiling point of the solution.

The temperature coei'ilcients or both 01' the above-described reactions are appreciable. The caustic solution can be regenerated by the air and/or other oxidizing gas at ordinary temperatures but elevated temperatures are to be preierred since the time of regeneration and, hence, the amount of air used is reduced.

The disulphides which are formed may be removed rrom the caustic, before re-use of the latter for extracting further quantities oi mercaptans, by allowing the caustic solution to stand for a little time whereupon the disulphides will separate as a layer to the top of the solution, from which it may be removed by decantation. Or they may be removed by washing the solution with gas oil or other liquid in which the disulphides are very miscible. The sodium hydroxide is then ready for re-use.

The concentration of the sodium hydroxide solution used in the process may vary over wide limits, although 2 to 5 normal (8 to 20 per cent) is preferred, since the hydroxide ion concentration (and hence mercaptan removal) is greatest in this range.

Other suitable alkaline solutions, such as potassium hydroxide, "calcium hydroxide, sodium zincate, etc., may be substituted for the sodium hydroxide.

The oxidation of the mercaptides may be accelerated by passing the oxidizing gas into the alkaline solution in finely subdivided form.

1,cos,ees

Porous materials, such as used for filtering liquors, serve excellently for breaking up the air stream into fine bubbles.

The process is applicable to any petroleum oil, liquefied petroleum gas, or gas containing mercaptans.

The alkyl disulphides may be recovered from the absorbent gas oil by ordinary distillation or by flash evaporation. Large quantities of the disulphides can thus be obtained.

The regeneration process may preferably be practiced in conjunction with the usual sodium hydroxide wash to remove hydrogen sulphide from the oil or gas. In other words, the sodium hydroxide treatment may be considered as comprising two separate stages, the first being for hydrogen sulphide removal and the second for mercaptan removal.

What is claimed and desired to be secured by Letters Patent is:

1. In a process for desulphurizing mercaptanbearing oil, wherein the oil is subjected to the action or an aqueous alkaline wash, the steps of separating the spent alkaline wash from the oil, treating the spent wash with a gas containing free oxygen, said treatment beingeffected at elevated temperatures approximating the boiling point of the wash, whereby the lower mercaptides in the wash are converted into disulphides and the higher boiling mercaptans are volatilized and the alkaline wash regenerated, and washing the said disulphides out of the regenerated alkaline wash by means 01' gas oil and the like.

2. A process for desulphurizing mercaptanbearing oil which comprises contacting the oil with an aqueous caustic wash, separating the mercaptide-containing caustic wash from the oil, treating said mercaptide-containing caustic wash with a gas containing tree oxygen, said treatment being effected at elevated temperature approximating the boiling point or the caustic wash, whereby the lower mercaptides in the wash are converted to disulphides and the higher boiling mercaptans are volatilized and the caustic wash regenerated, removing the said disulphides from the regenerated caustic wash, and returning the regenerated wash to the system for re-use.

3. In a process for desulphurizing mercaptanbearing 011, wherein the oil is subjected to the action or an aqueous alkaline wash, the steps of separating the spent alkaline wash from the oil, treating the spent wash with a gas containing tree oxygen, said treatment being effected at elevated temperatures approximating the boiling point 01 the wash, whereby the lower mercaptides in the wash are converted into disulphides and the higher boiling mercaptans are volatilized and the alkaline wash regenerated, and removing the said disulphides from the regenerated solution by settling and decantation.

LOVELL V. CHANEY. WALTER A. SCHULZE. 

